The present invention concerns a process for preparing silanes or siloxanes (hereinafter abbreviated as sil(ox)ane) having carboxyl functionalities. This process is characterized in that a hydrogen silane or hydrogen siloxane compound is reacted with a tertiary butyl ester having an olefinic double bond, e.g. t-butyl methacrylate, in the presence of a transition metal catalyst and subsequent thereto the resulting sil(ox)ane-bonded tertiary butyl ester group is converted into the corresponding carboxyl group with elimination of isobutene. Particular advantages of this process are the solvent-free, single-stage synthesis, the high selectivities in respect of the yield of free carboxyl groups and that commercially available starting compounds are used.
Methods for preparing carboxyl-functional silanes or siloxanes are known and described, for example, in W. Noll: "Chemie und Technologie der Silicone" p. 142 ff. (2nd edition, Verlag Chemie, Weinheim 1968). Thus, SiH bonds can undergo hydrosilylation reactions with unsaturated carboxyl groups in the presence of known catalysts such as platinum compounds. The selectivity of such reactions is, however, very low, since the condensation of SiH and the acidic proton of the acid group with elimination of hydrogen takes place as a significant side-reaction and Si--O--C-linked structures are obtained. The hydrosilylation of acrylonitrile and subsequent acid-catalyzed hydrolysis likewise leads to carboxyl-functional sil(ox)anes. The disadvantage of this method is the use of the toxicologically problematical acrylonitrile and the presence of water which can result, particularly in the case of the siloxane derivatives, in undesired emulsion problems in the separation. In addition, a quantitative yield of free carboxyl groups can be obtained only with great difficulty. The same problems occur in the hydrolysis of addition products of unsaturated esters such as methyl methacrylate or trimethylsilyl methacrylate to sil(ox) anes.
Methods for introducing the carboxyl groups via Grignard processes (magnesium and CO.sub.2 with, for example, Si--CH.sub.2 --Cl) or from carbonyl compounds having activated CH groups (malonic ester syntheses using haloalkylsiloxanes, e.g. Si--CH.sub.2 --Cl, and subsequent hydrolysis and decarboxylation) are not very suitable as industrial processes, since high demands have to be made of the purity of the starting materials and of the process equipment.
For the preparation of carboxyl-functional sil(ox)anes, it would be a considerable advantage if a solvent-free, single-stage and highly selective synthesis could be found.